This invention relates to valve control apparatuses and, in particular, to valve control apparatuses for diesel engine compression release brakes.
Compression release brakes are used to slow diesel powered vehicles such as large tractor trailer units. These brakes work by releasing compressed gases from each cylinder near top dead center of each compression stroke. This removes the rebound effect whereby the compressed gases would tend to drive the piston downwardly and thereby counter the braking effect otherwise created when the pistons compress gases during the compression stroke. Engine brakes are normally operated when a vehicle is coasting downhill and the fuel supply to the engine has been cut off. Wear on the wheel brakes is reduced since an engine brake significantly reduces the braking contribution required from the wheel brakes.
At least one exhaust valve on each cylinder is cracked open just before top dead center of each compression stroke when the brake is operational. Some mechanism must be provided, therefore, to open each exhaust valve twice during each engine cycle. The normal exhaust valve opening occurs during the exhaust stroke when the piston is moving upwardly towards the cylinder head. The second exhaust valve opening occurs during braking operation near the top dead center position at the end of the compression stroke. Various mechanisms have been devised to selectively crack open each exhaust valve the second time during each engine cycle. In many engines, for example, a fuel injector mechanism is used to crack open each exhaust valve at the required time. However such a mechanism is not available, nor suitable for all types of engines. Accordingly, alternative mechanisms have been devised.
One problem with such prior art engine brakes is that the normal operation of the exhaust valve is affected during brake operation. Clearance between the cam follower and camshaft is effectively reduced during brake operation. This means that the first lobe on the camshaft opens the exhaust valve further than normal for the exhaust stroke during exhaust brake operation. In some cases it is necessary to provide recesses in the pistons so that the exhaust valves do not strike the pistons when the brake is operational. These recesses, and the abnormally extended exhaust valves, interfere with optimal engine design from the point of view of other considerations such as emission controls.
Another problem with such prior art engine brakes is that the exhaust valve overlap at top dead center may be increased during brake operation. This means that exhaust gas energy is lost from the exhaust manifold to the inlet stroke of the cylinder. Recovering the lost energy would be beneficial in order to drive the turbocharger to supercharge the compression stroke.
It is an object of the invention to provide an improved valve control apparatus which overcomes the disadvantages associated with the prior art.
It is also an object of the invention to provide an improved valve control apparatus which allows a camshaft to selectively open each exhaust valve near top dead center of each compression stroke, for engine braking purposes, without interfering with normal maximum lift and closing of each exhaust valve on each exhaust stroke.
Is a further object of the invention to provide an improved valve control apparatus which is rugged and economical in construction and reliable during operation.
There is provided, according to one aspect of the invention, a valve control apparatus for an internal combustion engine having a valve and a camshaft. The camshaft has an axis of rotation, a first raised portion and a second lobe. The second raised portion is angularly spaced-apart about the axis from the first raised portion. The first raised portion extends further from the axis of rotation than the second raised portion. The apparatus includes a follower which is operatively engagable with the camshaft and the valve. The follower is positioned to operatively engage the first raised portion on each revolution of the camshaft and to open the valve a first time on each revolution of the camshaft. There is a mechanism for selectively changing operative clearance between the follower and at least one of the camshaft and the valve. The mechanism selectively reduces the clearance on each revolution of the camshaft after the valve is opened by the first raised portion. The follower operatively engages the second raised portion and opens the valve a second time on each revolution of the camshaft when the clearance is so reduced. The mechanism has a device which increases the clearance on each revolution of the camshaft after the valve is opened the second time and before the valve is fully opened by the first raised portion again. The device may be triggered by the camshaft.
The follower may be a rocker arm assembly and alternatively the device may be triggered between relative movement between a portion of the rocker arm assembly and the valve of the engine.
There is provided, according to another aspect of the invention, an internal combustion apparatus which includes an engine having a plurality of cylinders. Each cylinder has a valve. There is a camshaft having an axis of rotation, a first raised portion and a second raised portion. The second raised portion is angularly spaced-apart about the axis from the first raised portion. A follower is operative engagable with the camshaft and with the valve of said each cylinder. The follower has a first operational mode where the first raised portion operatively engages the follower on each revolution of the camshaft to open the valve a first time and where the second raised portion operatively clears the follower on each revolution of the camshaft without actuating the valve. There is a mechanism for selectively putting the follower in a second operational mode where the second raised portion operatively engages the follower to open the valve a second time on each revolution of the camshaft. The mechanism puts the follower in the second operational mode on each revolution of the camshaft before the second raised portion is fully aligned with the follower. The mechanism has a device for returning the follower to the first operational mode after the valve is opened by the second raised portion and before the first raised portion fully operatively engages the follower. Maximum opening and normal closing of the valve by the first raised portion is unaffected when the mechanism selectively puts the follower in the second operational mode. The device may be triggered by the camshaft.
Alternatively, where the follower is a rocker arm assembly, the device may be triggered by relative movement between a portion of the rocker arm assembly and the valve.
There is provided, according to a further aspect of the invention a method of controlling valve actuation for an internal combustion engine having a valve and a camshaft. The method comprises the steps of providing a mechanism operatively connection the valve and the camshaft. Two raised portions are provided on the camshaft for the valve. A first raised portion operatively engages the mechanism to open the valve a first time on each revolution of the camshaft. The mechanism is selectively adjusted a first time on each revolution of the camshaft after the first raised portion passes the mechanism so the mechanism engages a second said raised portion to open the valve a second time on each revolution of the camshaft. The mechanism is adjusted a second time on each revolution of the camshaft after the second raised portion opens the valve so the maximum opening and normal closing of the valve by the first raised portion is unaffected after the mechanism is selectively adjusted the second time. The mechanism may be adjusted the second time by a device which contacts the camshaft.
Alternatively, where the follower is a rocker arm assembly, the mechanism may be adjusted the second time by relative movement between a portion of the rocker arm assembly and the valve of the engine.